Simone, the wife of Jonathan Trevanny, the ordinary man whom Tom Ripley makes into a murderer in Ripley's Game, has no doubt what to call her husband's seducer: "that monster". Materialistic, unscrupulous and manipulative, Ripley certainly qualifies as a bad man. He lives in high style off theft and forgery. In the course of Ripley's Game, he kills five men - one by garrotting, the other four by various kinds of bludgeoning - without hesitation or guilt. He also corrupts the previously innocent Jonathan. Yet Jonathan himself reacts to his wife's horror by thinking, "Tom was not really such a monster". As Ripley knows, "once people got to know him... people liked him". The reader does too.

Patricia Highsmith's part-time criminal and occasional murderer is usually described as an anti-hero. The term is disputed, for sometimes it is applied to blundering, unheroic protagonists (Schweik in The Good Soldier Schweik, say). Yet it usefully characterises a central character who draws us into sympathy despite doing things that should appal us. An anti-hero is not the same as an alluring villain. The anti-hero takes possession of a narrative without any effective opposition. Villains are set against representatives of good. In the great majority of cases, a villain exists to be defeated. But there is no one to defeat Ripley. No one, except the reader, finds him out.

In all Highsmith's Ripley novels, we hear Ripley thinking from the very beginning. Characteristically, the first thought that he has in this novel is self-critical: he regrets speaking to a criminal associate "in a stuffy, pontificating way". Ripley is all for candour and against pretension. He has values that seem almost ethical.

He is psychologically acute, using his knowledge of Jonathan's terminal illness to exploit him. Like most anti-heroes, he has discernment rather than principles. Nabokov's Humbert Humbert in Lolita is an outstanding example. Nabokov's paedophile narrator is a man of wit and fine sensitivities. Like Ripley, he knows about art and beauty and good food.

Ripley lives in an impeccably furnished chateau. He appreciates classical music. When a mafia hood first intrudes on his home life, he is seated at the harpsichord, "playing the bass of a Goldberg variation, trying to get the fingering in his head and in his hand". Above all, he is "civilised" (one of his favourite terms of approbation).

He is also gifted with a sense of propriety. He disapproves of gambling and prostitution. He is earnestly concerned that Jonathan be paid in full for carrying out his killings. He suggests, "in jest", to his criminal friend Reeves that a fake medical report indicating that Jonathan is close to death might help secure his cooperation, but then realises that "Reeves was the type to have tried it - a dirty, humourless trick". For a moment we are inside the logic of Ripley's mind, feeling distaste for a scheme he has himself mooted.

An odd monster, then. Partly, the anti-hero offers us the frisson of taboo-breaking. Ripley's Game is about how a "basically decent" man (Ripley's own description of Jonathan) can be persuaded into murder. Yet Highsmith also has a curiously moral purpose. She has invented a character who lacks only a conscience. By creating such an engaging anti-type, she shows what an amoral life might be like.

· John Mullan is senior lecturer in English at University College London

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